The present invention relates to treating glass sheets, and particularly refers to a method of reducing vents that result in breakage during thermal treatment commonly associated with tempering of glass sheets. Glass sheets are scored when they are cut and/or drilled to provide holes extending through the thickness of the glass. The scored regions usually have vents associated therewith.
When glass sheets are tempered, they are heated to above the strain point of the glass, and even as high as a temperature approaching the glass softening point. After the glass attains a temperature sufficient for tempering, it is chilled rapidly. Stresses are established temporarily in the glass during this rapid cooling. These stresses may become so severe as to cause the glass sheet to fracture during the tempering operation. Glass fracturing during tempering is frequently observed in the vicinity of vents near those regions that are scored when the glass is cut, such as for drilling holes through the glass.
Many fabricated products of glass require thermal treatment involving heating the sheet to between its strain point and its melting point as part of their fabrication. Among the many articles of commerce that require such thermal treatment are tempered and partly tempered windows, lenses, cover plates and automotive and other vehicle glazing closures, such as windshields, backlights, vent panes, And sidelights.
Many movable glazing closures for present day vehicles must be drilled to produce holes that receive actuating rods associated with motors or cranks or other actuating mechanisms for opening and closing the glazing closures. Also, many of the articles of commerce fabricated from glass sheets require pre-cutting to irregular outlines. Unless considerable care is taken to seam the edges of the pre-cut sheets or to smooth the drilled portion formed into a hole, vents form along the edge of the glass periphery around the drilled portion. Such vents sometimes result in breakage during the thermal treatment. This breakage increases in frequency when the glass sheets so fabricated have a nominal thickness of 5/32 inch (4 millimeters) and less instead of thicker glass sheets that were used for similar parts in the prior art.
The prior art recognizes the fact that vents are associated with glass breakage during the tempering operation involving the thermal treatment described previously. It is well known that a glass sheet to be tempered adequately must be heated to a temperature sufficient to lower the viscosity of the glass sheet to enable it to flow sufficiently when chilled rapidly to establish a surface region stressed in compression surrounding an interior region stressed in tension. If the glass sheet is heated to a temperature above the optimum temperature for tempering, the glass sheet tends to lose its shape before it is subjected to the rapid cooling step. The lower the maximum temperature to which the glass sheet is heated, the greater is the likelihood that the glass sheet will fracture, particularly when temporary tension stresses arise during thermal treatment associated with tempering that cause a vent to develop into glass breakage. The need to heat each glass sheet to a maximum temperature within a narrow temperature range suitable for tempering is difficult to obtain in a commercial mass production operation where successive glass sheets precut to an identical outline of a production pattern differ in thickness and in distributions of stress around the scored regions so that a uniform time-temperature cycle applied to each glass sheet in a series of identical production pattern outlines does not necessarily avoid breakage of some of the treated glass sheets, particularly when the sheets are scored to produce holes through their thickness.
In the past, several different techniques have been suggested for reducing the losses due to breakage originating in the vicinity of vents in thermally treated glass articles at scored regions. These include providing a different thermal treatment for the region likely to be the source of breakage than the remainder of the glass sheet undergoing thermal treatment to develop a temper. Some of these techniques include blocking or insulating heat locally to provide a reduction in damage in the blocked or insulated region; applying a heat absorbing and re-radiating material in the vicinity of the scored region to cause the glass to selectively heal any vents by increasing the local temperature in the vicinity of the scored region; using an insulating material to prevent heat loss and reduce temporary stress levels locally during a thermal treatment involved in tempering, and selectively applying a heat source locally to the vicinity of an aperture during thermal treatment involved in tempering. In addition, it has been recognized that edges, as well as drilled holes, become weakened as a result of scoring and cutting to divide larger sheets into smaller sheets, and these have been strengthened by selective seaming, by localized heating, by application of fluoride-containing compounds and by applying materials having a low coefficient of thermal expansion compared to the glass such as a glassy frit having a melting point less than the rest of the glass sheet along seamed edges that require strengthening.